1. Field of the Invention
The invention relates to a method and a system for synthesizing a set of controllable light beams.
2. Description of the Related Art
It is well known to form an image on an illuminated surface of a body by absorption or blocking of energy of an illuminating beam. For example in an overhead projector, an overhead transparent absorbs or blocks part of the light beam of the projector whereby a large image of an overhead is formed on a screen. However, this result in a loss of light intensity since part of the emitted light from such an image forming system is reflected or absorbed.
To avoid loss of energy causing, e.g., loss of light intensity of the synthesized intensity pattern, power dissipation generating heat in components of the system, etc., methods and systems have been developed wherein the phase of a light beam is modulated instead of the amplitude or intensity of the light beam, since modulation of the phase of the light beam does not result in loss of energy. The phase modulation is followed by conversion of phase modulation into amplitude or intensity modulation.
Imaging methods and systems may also be used in connection with phase modulation. These methods and systems are characterized by the fact that the intensity of a point of a picture formed by conversion of phase modulation into intensity modulation will depend upon the phase modulation value of one point of the phase modulator only since this point is imaged onto the picture point in question by the imaging system.
This one-to-one relationship makes the design of phase modulators in these systems simple. Methods and systems of this kind are named phase contrast imaging methods and systems.
Phase contrast imaging methods were originally developed within the field of microscopy. Many objects of interest in microscopy are largely transparent, thus absorbing little or no light. When light passes through such an object, the predominant effect is the generation of a spatially varying phase shift which can not be seen by a human since the eye of a human responds to light intensity and colour and does not respond to the phase of light.
In 1935, Fritz Zernik proposed a phase contrast technique which relies on spatial-filtering principles and has the advantage that the observed intensity is linearly related to the phase shift introduced by the object.
A generalized phase contrast imaging method and system for synthesizing a prescribed intensity pattern is disclosed in WO 96/34307, which is hereby incorporated by reference. The generalized method is not based on the so-called Zernike approximation that the phase shift is less than 1 radian. An improved method is provided without this approximation and based on imaging with a simple one-to-one mapping of resolution elements or pixels of a spatial phase modulator onto resolution elements of the generated intensity pattern.
One application of the present invention relates to manipulation of micro-objects, such as micro-components, biological cells, etc. using electromagnetic gradient forces.
It is well-known that in a strongly focused laser beam having an approximately Gaussian intensity profile, radiation pressure scattering and gradient force components are combined to provide a point of stable equilibrium located close to the focus of the laser beam. Scattering force is proportional to optical intensity and acts in the direction of the incident laser light. Gradient force is proportional to the optical intensity and points in the direction of the intensity gradient.
This effect is utilized in so-called optical tweezers or optical traps wherein the optical gradient forces in a focused light beam trap a small micro-object at the focal point of the light beam. The micro-object is typically immersed in a fluid medium whose refractive index is smaller than that of the micro-object. The optical tweezer technique has been generalized to enable manipulation of reflecting, absorbing and low dielectric constant micro-objects. Typically, a Gaussian beam is used for trapping of micro-objects with a refractive index that is higher than the refractive index of its surroundings while a donut beam is used for trapping of a micro-object with refractive index that is lower than the refractive index of its surroundings.